In drilling a borehole, such as for the recovery of hydrocarbons or for other applications, it is conventional practice to connect a drill bit on the lower end of an assembly of drill pipe sections that are connected end-to-end so as to form a drill string. The bit is rotated by rotating the drill string at the surface or by actuation of downhole motors or turbines, or by both methods. With weight applied to the drill string, the rotating bit engages the earthen formation causing the bit to cut through the formation material by either abrasion, fracturing, or shearing action, or through a combination of these and/or other cutting methods, thereby forming a borehole.
Many different types of drill bits have been developed and found useful in drilling such boreholes. Two common types of drill bits are roller cone bits and fixed cutter (or rotary drag) bits. Most fixed cutter bit designs include a plurality of blades angularly spaced about the bit face. The blades project radially outward from the bit body and form flow channels therebetween. In addition, cutting elements are typically grouped and mounted on several blades in radially extending rows. The configuration or layout of the cutting elements on the blades may vary.
The cutting elements on the blades of a fixed cutter bit are typically formed of extremely hard materials. In atypical fixed cutter bit, each cutting element includes an elongate and generally cylindrical tungsten carbide substrate that is received and secured in a pocket formed in the surface of one of the blades. The cutting elements typically include a hard cutting layer of polycrystalline diamond (PCD) or other superabrasive materials such as thermally stable diamond or polycrystalline cubic boron nitride. These cutting elements are designed to shear formations that range from soft to medium hard. For convenience, as used herein, reference to “PDC bit” or “PDC cutters” refers to a fixed cutter bit or cutting element employing a hard cutting layer of polycrystalline diamond or other superabrasive materials.
Without regard to the type of bit, the cost of drilling a borehole is proportional to the length of time it takes to drill the borehole to the desired depth and location. The drilling time is affected by the number of times the drill bit is changed in order to reach the targeted formation, as each time the bit is changed, the entire drill string, which may be miles long, is retrieved from the borehole section by section. Once the drill string has been retrieved and the new bit installed, the bit is lowered to the bottom of the borehole on the drill string, which again is constructed section by section. This process, known as a trip of the drill string, often requires considerable time, effort, and expense.
The length of time that a drill bit may be used before it is changed depends upon its rate of penetration (ROP), as well as its durability or ability to maintain a high or acceptable ROP. Specifically, ROP is the rate that a drill bit penetrates a given subterranean formation. Drill bit designs are modified to improve ROP in specific formations so as to reduce drilling time, and thus, cost.
Once a desired formation is reached in the borehole, a core sample of the formation may be extracted for analysis. A hollow coring bit is often employed to extract a core sample from the formation. Once the core sample has been transported from the borehole to the surface, the sample may be used to analyze and test, for example, permeability, porosity, composition, or other geological properties of the formation. Conventional coring methods require retrieval of the drill string from the borehole, replacement of the drill bit with a coring bit, and lowering of the coring bit into the borehole on the drill string in order to retrieve a core sample, which is then removed to the surface for analysis.